This application is a 371 of PCT/GB96/03068, filed Dec. 12, 1996 and claims prior to foreign application numbers UK/9525455.3, filed Dec. 13, 1995 and UK/9606552.9, filed Mar. 28, 1996.
This invention relates to antifungal proteins, processes for their manufacture and use, and DNA sequences encoding them.
In this context, antifungal proteins are defined as proteins or peptides possessing antifungal activity. Activity includes a range of antagonistic effects such as partial inhibition or death.
A wide range of antifungal proteins with activity against plant pathogenic fungi have been isolated from certain plant species. We have previously described a class of antifungal proteins capable of isolation from radish and other plant species. These proteins are described in the following publications which are specifically incorporated herein by reference: International Patent Application Publication Number W093/05153 published Mar. 18, 1993; Terras FRG et al, 1992, J Biol Chem, 267:15301-15309; Terras et al, 1993, FEBS Lett, 316:233-240; Terras et al, 1995, Plant Cell, 7:573-588. The class includes Rs-AFP1 (antifungal protein 1), Rs-AFP2, Rs-AFP3 and Rs-AFP4 from Raphanus sativus and homologous proteins such as Bn-AFP1 and Bn-AFP2 from Brassica napus, Br-AFP1 and Br-AFP2 from Brassica rapa, Sa-AFP1 and Sa-AFP2 from Sinapis alba, At-AFP1 from Arabidopsis thaliana, Dm-AMP1 and Dm-AMP2 from Dahlia merckii, Cb-AMP1 and Cb-AMP2 from Cnicus benedictus, Lc-AFP from Lathyrus cicera, Ct-AMP1 and Ct-AMP2 from Clitoria ternatea. The proteins specifically inhibit a range of fungi and may be used as fungicides for agricultural or pharmaceutical or preservative purposes.
It has been proposed that this class of antifungal proteins should be named plant defensins (Terras F.R.G. et al 1995, Plant Cell, 7 573-583) and these proteins have in common a similar motif of conserved cysteines and glycines (Broekaert W. F. et al 1995, Plant Physiol. 108 1353-1358).
FIG. 1 shows the amino acid sequences of the protein Rs-AFP1 (SEQ ID NO: 34) and the substantially homologous proteins Rs-AFP2 (SEQ ID NO: 35), Rs-AFP3 (SEQ ID NO: 36), Rs-AFP4 (SEQ ID NO: 37), Br-AFP1 (SEQ ID NO: 38), Br-AFP2 (SEQ ID NO: 39), Bn-AFP1 (SEQ ID NO: 40), Bn-AFP2 (SEQ ID NO: 41), Sa-AFP1 (SEQ ID NO: 42), Sa-AFP2 (SEQ ID NO: 43) and At-AFP1 (SEQ ID NO: 44) which are small 5 kDa polypeptides that are highly basic and rich in cysteine. FIG. 1 numbers the positions of the amino acid residues: the dash (-) at the start of the Rs-AFP3 (SEQ ID NO: 36) sequence indicates a gap introduced for maximum alignment. The sequences shown for Br-AFP1 (SEQ ID NO: 38), Br-AFP2 (SEQ ID NO: 39), Bn-AFP1 (SEQ ID NO: 40), Bn-AFP2 (SEQ ID NO: 41), Sa-AFP1 (SEQ ID NO: 42), Sa-AFP2 (SEQ ID NO: 43) and At-AFP1 (SEQ ID NO: 44) are not complete: only the N-terminal sequences are shown. The question mark (?) in the Br-AFP2 (SEQ ID NO: 39) sequence indicates a non-standard amino acid which the sequencing could not assign and which is thought to be a post-translational modification on one of the standard amino acid residues.
Further examples of antifungal plant defensins are described in International Patent Application Publication Number W095/18229 published Jul. 6, 1995 which is specifically incorporated herein by reference. These examples include Hs-AFP1, an antifungal protein capable of isolation from seeds of Heuchera species and Ah-AMP1, an antimicrobial protein capable of isolation from seeds of Aesculus hippocastanum. The proteins specifically inhibit a range of fungi and may be used as fungicides for agricultural or pharmaceutical or preservative purposes.
FIG. 9 shows the amino acid sequences of the proteins Hs-AFP1 and Ah-AMP1. FIG. 9 numbers the positions of the amino acid residues. The Hs-AFP1 sequence shows 48% sequence identity with Rs-AFP1. The Ah-AMP1 sequence shows 54% sequence identity with Rs-AFP1. Hs-AFP1 shows 52% identity to Ah-AMP1 on the amino acid sequence level.
The primary structures of the two antifungal protein isoforms capable of isolation from radish seeds, Rs-AFP1 and Rs-AFP2, only differ at two positions: the glutamic acid residue (E) at position 5 in Rs-AFP1 is a glutamine residue (Q) in Rs-AFP2, and the asparagine residue (N) at position 27 in Rs-AFP1 is substituted by an arginine residue (R) in Rs-AFP2. As a result, Rs-AFP2 has a higher net positive charge (+2) at physiological pH. Although both Rs-AFPs are 94% identical at the amino acid sequence level, Rs-AFP2 is two- to thirty-fold more active than Rs-AFP1on various fungi and shows an increased salt-tolerence. The proteins Rs-AFP3 and Rs-AFP4 are found in radish leaves following localized fungal infection. The induced leaf proteins are homologous to Rs-AFP1 and Rs-AFP2 and exert similar antifungal activity in vitro.
The cDNA encoding Rs-AFP1 encodes a preprotein with a signal peptide followed by the mature protein. The cDNA sequence is shown in FIG. 2. Saccharomyces cerevisiae can be used as a vector for the production and secretion of Rs-AFP2 (Vilas Alves et al, FEBS Lett, 1994, 348:228-232). Plant-derivable xe2x80x9cwild-typexe2x80x9d Rs-AFP2 can be correctly processed and secreted by yeast when expressed as a N-terminal fusion to the yeast mating factor xcex11 (MFxcex11) preprosequence. The Rs-AFP2 protein does not have adverse effects on yeast even at concentrations as high as 500 xcexcg/ml.
We now provide new potent antifungal peptides based on the structure of the Rs-AFPs and related plant defensins.
According to the first aspect of the present invention there is provided an antifungal peptide which comprises at least six amino acid residues identical to a run of amino acid residues found between position 21 and position 51 of the Rs-AFP2 sequence shown in FIG. 1 or of substantially homologous protein sequences.
Proteins which are substantially homologous to the Rs-AFP2 protein include the proteins Rs-AFP1, Rs-AFP3, Rs-AFP4, Br-AFP1, Br-AFP2, Bn-AFP1, Bn-AFP2, Sa-AFP1, Sa-AFP2 and At-AFP1 shown in FIG. 1 and Hs-AFP2, Ah-AMP1 and Dm-AMP1 shown in FIG. 9. Proteins which are substantially homologous have an amino acid sequence with at least 40% sequence identity to any of the sequences shown in FIGS. 1 and 9, and preferably at least 60% identity; and most preferably at least 80% identity.
Antifungal peptides according to the invention include especially peptides derived from the beta-2 strand/turn/beta-3 strand region of Rs-AFP2 and substantially homologous antifungal protein sequences. Preferred antifungal peptides according to the invention include the 6-mer, 9-mer and 12-mer, 13-mer, 14-mer, 15-mer, 16-mer, 17-mer, 18-mer, 19-mer and 20-mer, and most especially the 18-mer, 19-mer and 20-mer peptides described in the accompanying examples, figures and tables especially Example 11 and FIGS. 10 to 13.
Antifungal peptides according to the invention include the following peptides:
a peptide comprising fifteen amino acid residues identical to a run of fifteen amino acid residues found between position 21 and position 35 of the Rs-AFP2 sequence shown in FIG. 1 and having the sequence: CKNQCIRLEKARHGS;
a peptide comprising fifteen amino acid residues identical to a run of fifteen amino acid residues found between position 25 and position 39 of the Rs-AFP2 sequence shown in FIG. 1 and having the sequence: CIRLEKARHGSCNYV;
a peptide comprising fifteen amino acid residues identical to a run of fifteen amino acid residues found between position 29 and position 43 of the Rs-AFP2 sequence shown in FIG. 1 and having the sequence: EKARHGSCNYVFPAH;
a peptide comprising fifteen amino acid residues identical to a run of fifteen amino acid residues found between position 33 and position 47 of the Rs-AFP2 sequence shown in FIG. 1 and having the sequence: HGSCNYVFPAHKCIC;
a peptide comprising ten amino acid residues identical to a run of ten amino acid residues found between position 36 and position 45 of the Rs-AFP2 sequence shown in FIG. 1 and having the sequence: CNYVFPAHKC;
a peptide comprising six amino acid residues identical to a run of six amino acid residues found between position 40 and position 45 of the Rs-AFP2 sequence shown in FIG. 1 and having the sequence: FPAHKC;
a peptide comprising six amino acid residues identical to a run of six amino acid residues found between position 42 and position 47 of the Rs-AFP2 sequence shown in FIG. 1 and having the sequence: AHKCIC;
a peptide comprising six amino acid residues identical to a run of six amino acid residues found between position 43 and position 48 of the Rs-AFP2 sequence shown in FIG. 1 and having the sequence: HKCICY;
a peptide comprising nine amino acid residues identical to a run of nine amino acid residues found between position 24 and position 32 of the Rs-AFP2 sequence shown in FIG. 1 and having the sequence: QCIRLEKAR;
a peptide comprising nine amino acid residues identical to a run of nine amino acid residues found between position 25 and position 33 of the Rs-AFP2 sequence shown in FIG. 1 and having the sequence: CIRLEKARH;
a peptide comprising nine amino acid residues identical to a run of nine amino acid residues found between position 32 and position 40 of the Rs-AFP2 sequence shown in FIG. 1 and having the sequence: RHGSCNYVF;
a peptide comprising nine amino acid residues identical to a run of nine amino acid residues found between position 36 and position 44 of the Rs-AFP2 sequence shown in FIG. 1 and having the sequence: CNYVFPAHK;
a peptide comprising nine amino acid residues identical to a run of nine amino acid residues found between position 40 and position 48 of the Rs-AFP2 sequence shown in FIG. 1 and having the sequence: FPAHKCICY;
a peptide comprising nine amino acid residues identical to a run of nine amino acid residues found between position 41 and position 49 of the Rs-AFP2 sequence shown in FIG. 1 and having the sequence: PAHKCICYF;
a peptide comprising nine amino acid residues identical to a run of nine amino acid residues found between position 42 and position 50 of the Rs-AFP2 sequence shown in FIG. 1 and having the sequence: AHKCICYFP;
a peptide comprising nine amino acid residues identical to a run of nine amino acid residues found between position 43 and position 51 of the Rs-AFP2 sequence shown in FIG. 1 and having the sequence: HKCICYFPC;
a peptide comprising twelve amino acid residues identical to a run of twelve amino acid residues found between position 25 and position 36 of the Rs-AFP2 sequence shown in FIG. 1 and having the sequence: CIRLEKARHGSC;
a peptide comprising twelve amino acid residues identical to a run of twelve amino acid residues found between position 29 and position 40 of the Rs-AFP2 sequence shown in FIG. 1 and having the sequence: EKARHGSCNYVF;
a peptide comprising twelve amino acid residues identical to a run of twelve amino acid residues found between position 30 and position 41 of the Rs-AFP2 sequence shown in FIG. 1 and having the sequence: KARHGSCNYVFP;
a peptide comprising twelve amino acid residues identical to a run of twelve amino acid residues found between position 32 and position 43 of the Rs-AFP2 sequence shown in FIG. 1 and having the sequence: RHGSCNYVFPAH;
a peptide comprising twelve amino acid residues identical to a run of twelve amino acid residues found between position 33 and position 44 of the Rs-AFP2 sequence shown in FIG. 1 and having the sequence: HGSCNYVFPAHK;
a peptide comprising nineteen amino acid residues identical to a run of nineteen amino acid residues found between position 31 and position 49 of the Rs-AFP2 sequence shown in FIG. 1 and having the sequence: ARHGSCNYVFPAHKCICYF.
We have found that the presence of an arginine residue at position 27 and a phenylalanine residue at position 40; a lysine residue at position 30 and a histidine residue at position 43 or an arginine residue at position 32 and a lysine residue at position 44 is particularly advantageous in Rs-AFP2 based peptides. We have also found that antifungal peptides based on the Rs-AFP2 sequence with an N-terminal amino acid selected from the group lysine at position 30, alanine at position 31, arginine at position 32 or histidine at position 33 and a C-terminal amino acid comprising either a tyrosine residue at position 48 or a phenylalanine residue at position 49 are particularly active. These antifungal peptides form a further embodiment of the invention.
The invention also provides an antifungal peptide which comprises at least six amino acid residues identical to a run of amino acid residues found between position 30 and position 48 of the Ah-AMP1 sequence or the Hs-AFP1 sequence shown in FIG. 9. Such antifungal peptides include a peptide comprising nineteen amino acid residues identical to the run of nineteen amino acid residues found between position 30 and position 48 of the Ah-AMP1 sequence shown in FIG. 9 and having the sequence: ASHGACHKRENHWKCFCYF. The invention also provides a peptide comprising nineteen amino acid residues found between position 30 and position 48 of the Dm-AMP1 sequence shown in FIG. 9 and having the sequence AAHGACHVRNGKHMCFCYF.
Peptides derived from the regions defined herein of the Rs-AFP plant defensins exhibit antifungal activity. Such peptides may be easier to synthesise than the full length plant defensin while retaining antifungal activity. DNA sequences encoding the peptides may also be more suitable for transformation into biological hosts.
An antifungal peptide according to the invention may be manufactured from its known amino acid sequence by chemical synthesis using a standard peptide synthesiser, or produced within a suitable organism (for example, a micro-organism or plant) by expression of recombinant DNA. The antifungal peptide is useful as a fungicide and may be used for agricultural or pharmaceutical or other applications. The antifungal peptide may be used in combination with one or more of the antifungal proteins or with one or more other antifungal peptides of the present invention. For example, an antifungal composition comprising one of the above-mentioned fifteen-mer peptides plus the Rs-AFP2 or Rs-AFP1 protein may show enhanced activity.
Knowledge of its primary structure enables manufacture of the antifungal peptide, or parts thereof, by chemical synthesis using a standard peptide synthesiser. It also enables production of DNA constructs encoding the antifungal peptide.
The invention further provides a DNA sequence encoding an antifungal peptide according to the invention. The DNA sequence may be predicted from the known amino acid sequence and DNA encoding the peptide may be manufactured using a standard nucleic acid synthesiser.
The DNA sequence encoding the antifungal peptide may be incorporated into a DNA construct or vector in combination with suitable regulatory sequences (promoter, terminator, transit peptide, etc). For some applications, the DNA sequence encoding the antifungal peptide may be inserted within a coding region expressing another protein to form an antifungal fusion protein or may be used to replace a domain of a protein to give that protein antifungal activity. The DNA sequence may be placed under the control of a homologous or heterologous promoter which may be a constitutive or an inducible promoter (stimulated by, for example, environmental conditions, presence of a pathogen, presence of a chemical). The transit peptide may be homologous or heterologous to the antifungal protein and will be chosen to ensure secretion to the desired organelle or to the extracellular space. The transit peptide is preferably that naturally associated with the antifungal protein of interest. Such a DNA construct may be cloned or transformed into a biological system which allows expression of the encoded peptide or an active part of the peptide. Suitable biological systems include micro-organisms (for example, bacteria such as Escherichia coli, Pseudomonas and endophytes such as Clavibacter xyli subsp. cynodontis (Cxc); yeast; viruses; bacteriophages; etc), cultured cells (such as insect cells, mammalian cells) and plants. In some cases, the expressed peptide may subsequently be extracted and isolated for use.
An antifungal peptide according to the invention is useful for combatting fungal diseases in plants. The invention further provides a process of combating fungi whereby they are exposed to an antifungal peptide according to the invention. The antifungal peptide may be used in the form of a composition.
For pharmaceutical applications, the antifungal peptide (including any product derived from it) may be used as a fungicide to treat mammalian infections (for example, to combat yeasts such as Candida).
An antifungal peptide (including any product derived from it) according to the invention may also be used as a preservative (for example, as a food additive).
For agricultural applications, the antifungal peptide may be used to improve the disease-resistance or disease-tolerance of crops either during the life of the plant or for post-harvest crop protection. Pathogens exposed to the peptides are inhibited. The antifungal peptide may eradicate a pathogen already established on the plant or may protect the plant from future pathogen attack. The eradicant effect of the peptide is particularly advantageous.
Exposure of a plant pathogen to an antifungal peptide may be achieved in various ways, for example:
(a) The isolated peptide may be applied to plant parts or to the soil or other growth medium surrounding the roots of the plants or to the seed of the plant before it is sown using standard agricultural techniques (such as spraying).
The peptide may have been extracted from plant tissue or chemically synthesised or extracted from micro-organisms genetically modified to express the peptide. The peptide may be applied to plants or to the plant growth medium in the form of a composition comprising the peptide in admixture with a solid or liquid diluent and optionally various adjuvants such as surface-active agents. Solid compositions may be in the form of dispersible powders, granules, or grains.
(b) A composition comprising a micro-organism genetically modified to express the antifungal peptide may be applied to a plant or the soil in which a plant grows.
(c) An endophyte genetically modified to express the antifungal peptide may be introduced into the plant tissue (for example, via a seed treatment process).
An endophyte is defined as a micro-organism having the ability to enter into non-pathogenic endosymbiotic relationships with a plant host. A method of endophyte-enhanced protection of plants has been described in a series of patent applications by Crop Genetics International Corporation (for example, International Application Publication Number W090/13224, European Patent Publication Number EP-125468-B1, International Application Publication Number W091/10363, International Application Publication Number W087/03303). The endophyte may be genetically modified to produce agricultural chemicals. International Patent Application Publication Number W094/16076 (ZENECA Limited) describes the use of endophytes which have been genetically modified to express a plant-derived antifungal peptide.
(d) DNA encoding an antifungal peptide may be introduced into the plant genome so that the peptide is expressed within the plant body (the DNA may be cDNA, genomic DNA or DNA manufactured using a standard nucleic acid synthesiser).
Exposure of a plant pathogen to an antifungal composition comprising an antifungal peptide plus an antifungal protein may be achieved by delivering the protein as well as the peptide as described above. For example, both one of the above-mentioned fifteen-mer peptides plus Rs-AFP2 or Rs-AFP1 could be simultaneously applied to plant parts or simultaneously expressed within the plant body.
Plant cells may be transformed with recombinant DNA constructs according to a variety of known methods (Agrobacterium Ti plasmids, electroporation, microinjection, microprojectile gun, etc). The transformed cells may then in suitable cases be regenerated into whole plants in which the new nuclear material is stably incorporated into the genome. Both transformed monocotyledonous and dicotyledonous plants may be obtained in this way, although the latter are usually more easy to regenerate. Some of the progeny of these primary transformants will inherit the recombinant DNA encoding the antifungal peptide(s).
The invention further provides a plant having improved resistance to a fungal pathogen and containing recombinant DNA which expresses an antifungal peptide according to the invention. Such a plant may be used as a parent in standard plant breeding crosses to develop hybrids and lines having improved fungal resistance.
Recombinant DNA is DNA, preferably heterologous, which has been introduced into the plant or its ancestors by transformation. The recombinant DNA encodes an antifungal peptide expressed for delivery to a site of pathogen attack (such as the leaves). The DNA may encode an active subunit of an antifungal peptide.
A pathogen may be any fungus growing on, in or near the plant. In this context, improved resistance is defined as enhanced tolerance to a fungal pathogen is when compared to a wild-type plant. Resistance may vary from a slight increase in tolerance to the effects of the pathogen (where the pathogen in partially inhibited) to total resistance so that the plant is unaffected by the presence of pathogen (where the pathogen is severely inhibited or killed). An increased level of resistance against a particular pathogen or resistance against a wider spectrum of pathogens may both constitute an improvement in resistance. Transgenic plants (or plants derived therefrom) showing improved resistance are selected following plant transformation or subsequent crossing.
Where the antifungal peptide is expressed within a transgenic plant or its progeny, the fungus is exposed to the peptide at the site of pathogen attack on the plant. In particular, by use of appropriate gene regulatory sequences, the peptide may be produced in vivo when and where it will be most effective. For example, the peptide may be produced within parts of the plant where it is not normally expressed in quantity but where disease resistance is important (such as in the leaves).
Examples of genetically modified plants which may be produced include field crops, cereals, fruit and vegetables such as: canola, sunflower, tobacco, sugarbeet, cotton, soya, maize, wheat, barley, rice, sorghum, tomatoes, mangoes, peaches, apples, pears, strawberries, bananas, melons, potatoes, carrot, lettuce, cabbage, onion.
We have surprisingly found that when the peptides according to the invention are mixed with full length Rs-AFP2 a synergistic effect is observed where the antifungal activity of the mixture is better than that observed with the protein or the peptide on its own.
In a further aspect the invention provides an antifungal composition comprising a peptide according to the invention and Rs-AFP1 or Rs-AFP2.
The invention also extends to DNA constructs encoding both the antifungal peptide and Rs-AFP1 or Rs-AFP2, and to the use of said peptide mixtures in antifungal compositions for pharmaceutical, agricultural, and preservative applications.